This invention relates to apparatus and methods for magnetron sputtering.
In U.S. Pat. No. 5,593,551 issued to Lai, the disclosure of which is incorporated herein by reference, there is described an apparatus and method for magnetron sputtering in which a first closed-loop magnet means is positioned adjacent to a dish-shaped sputter target back surface for creating a closed-loop magnetic tunnel to the front surface of the sputter target, the magnet means being in the form of a plurality of magnets surrounding a central axis of a dish-shaped sputter target for containing and guiding a plasma relative to a substrate. The improvement is a second closed-loop magnet means positioned around the sputter target perimeter and being comprised of a number of bucking magnets, which may be permanent magnets or electro magnets, which collectively provide a fixed field around the sputter target to thereby reduce or inhibit the spreading of the discharge track of the plasma beyond the edge of the target as the operating pressure of the sputtering system is reduced.
The second closed-loop magnet means solves the problem in that the voltage at which a magnetron operates is primarily a function of the ease of ionisation. It is, in turn, a function of the gas, its pressure, the applied voltage, the strength of the magnetic field and the ionisation losses from the plasma. As pressure is dropped so operating voltages rise until practical limits are reached e.g. the power supply or electrical connections to the target.
Extremely high voltages are undesirable as they cause the plasma to emit higher energy electromagnetic waves that are potentially dangerous. Therefore, work has concentrated upon reduction of losses from the plasma by e.g. operating the plasma in an enclosed area bounded by a bucking arrangement, as in the '551 patent. This allows magnetron operation at pressures of 0.1 mT to 1 mT. This low pressure operation is beneficial as a means of improving step coverage.
However, the design of this system and method of sputtering has the disadvantage that due to the confinement of the primary magnetic field by the secondary magnetic field, there is little or zero erosion of the target material around the periphery of the target. As a consequence, the periphery thereof can thereafter be contaminated by back scattered deposits of eroded material, such as atoms or molecules. This re-deposited material generally does not adhere well to the otherwise clean periphery of the target and can thereafter flake off to contaminate the substrate to which a thin film of the material is being applied or is to be applied.